This invention relates to aerators for inhibiting fouling of immersed filtering membranes.
Aeration is used with immersed membranes to scour the membranes and to disperse areas of tank water having increased concentrations of rejected solids from near the membranes. In particular, immersed membranes in bioreactors quickly become surrounded in sludge if intensive course bubble aeration is not provided.
U.S. Pat. Nos. 5,192,456 and 5,482,625, issued on Mar. 9, 1993 and Jan. 9, 1996 to Kubota Corporation, describe an air diffuser disposed below a set of membrane cartridges. A casing surrounds the air diffuser and the membrane cartridges, extending vertically from the bottom of the diffuser to the top of the membrane cartridges. In commercial embodiments, the diffuser is located about 1 m below the membrane cartridges and the diffusers provide a relatively small number of holes per square meter of horizontal cross-sectional area of the assembly of membrane cartridges. Air is supplied such that the air velocity and pressure loss through the holes of the diffusers is sufficient to prevent water or sludge from creeping into the holes of the diffuser. The casing and location of the diffuser below the membrane cartridges encourages the bubbles to become evenly dispersed by the time that they reach the membrane cartridges. The shroud and deep aerators increase both the equipment cost and the energy required to produce bubbles.
Another approach is described in U.S. Pat. No. 5,944,997, issued on Aug. 31, 1999 to Zenon Environmental Inc. In this patent, aerators are located directly below a set of membrane modules and no shroud is used but there are many more holesxe2x80x94about 150 holes per square meter of horizontal cross-sectional area of the assembly of membrane modules. Although the large number of holes provides well distributed bubbles, the air flow per hole is not sufficient to prevent tank water or sludge from creeping into the aerators around the perimeter of the holes. To prevent this tank water from leaving deposits in the aerator, the aerators are periodically flushed with a two-phase mixture of air and permeate. Although effective, this method involves an extensive grid of aerators to provide the large number of holes and additional equipment for flushing the aerators.
It is an object of the present invention to improve on the prior art. This object is met by the combination of features, steps or both found in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination with features described in other parts of this document.
In one aspect, the invention provides one or more aerators that move below an area to be aerated. The movement can take various forms such as a horizontal back and forth motion, rotation in a generally horizontal plane about a generally vertical axis or the sweep of a pendulum.
In one embodiment, the one or more aerators move at such a speed that it has substantially the equivalent effect of continuous aeration although bubbles may not be provided to a selected position for periods of up to a few seconds.
In another embodiment, each position within the range of area aerated by the one or more aerators experiences a pattern of air flow that alternates between an air-on condition and an air-off condition such that each position has aerated periods of time and a non-aerated periods of time. Such an aerator encourages transient flow to persist in the tank water which reduces the size of any dead zones in the membrane assembly and enhances the fouling inhibiting effect of aeration.
The minimum aerated period of time for each position may be about 0.5 seconds or more. The maximum non-aerated period of time for each position may be about 60 seconds or less or between about 15 to 30 seconds.
The air flow through each aerator hole may be sufficient to substantially prevent tank water from creeping into the aerator and yet aerate every position in the range of movement of the aerator with minimal submergence of the aerator. The number of holes may be about 50 or less, preferably 25 or less, per square meter of the horizontal area of membrane assembly to be aerated. With reasonable amounts of total airflow (averaged over time and the area to be aerated), the airflow through each hole can be made sufficiently large to keep tank water from creeping into the holes of the aerators.